TCP performance issues in wireless networks have been studied in many papers. Documents [3] and [2] provide overview proposals for improving TCP performance in wireless mobile networks.
Another TCP freeze principle was introduced also by document [1]. The authors of that paper proposed to “freeze” the TCP source for the duration of a handover by making the TCP receiver in the UE send a TCP ACK with 0 advertised window size (Zero Window Advertisement—ZWA) when the UE expects a handover to happen. The ZWA forces the TCP source to freeze all retransmit-timers and enter persist-mode. Upon finishing the handover, the TCP receiver in the UE sends triple ACKs with positive window size to make the source leave persist-mode and continue sending segments using the same RTO timer value and congestion window size used before the persist-mode operation.
Although both the solution proposed by document [1] and the one according to the present invention as described below uses the TCP freeze principle, there is a major difference between them regarding the placement of the functionality being responsible for sending the ZWA as document [1] proposes to implement it in the UE while, according to the present invention, it is proposed to implement it in the radio access nodes. The drawback of having the freeze functionality in the UE is that the UE has only limited information about the mobile network, thus it cannot properly predict handovers and network coverage holes and it is not able to handle issues caused by short term prioritization and degraded radio conditions of wireless transport network links.
Also, it requires the UE to implement a cross-layer functionality between the TCP layer of the application protocol stack and the RRC layer. Moreover, this solution cannot be influenced directly by the mobile network operator.
Therefore, the TCP freeze mechanism implemented in radio network nodes according to the present invention has an advantage with respect to the one implemented in UEs as it can provide a wider range and more accurate freeze triggers and it is transparent to the UE, with no standardization and implementation impact on UE manufacturers.
Document [6] proposes a solution that combines the TCP freeze principle and Mobile IP in the persistent TCP using Simple Freeze (PETS) framework for preventing TCP from disconnecting in mobile networks. The framework uses ICMP messages for monitoring link states and detecting link failures, upon which TCP flows are frozen by ZWAs for the duration of the failures. Thus, the termination of TCP connections can be avoided by PETS.
However, this solution cannot tackle the issues solved by the present invention (spurious timeouts due to increased RTT and packet losses in bad channel conditions, etc.) as the ICMP based link state detection is not suitable for that purpose. Also, the present invention does not depend on defining new ICMP messages for its operation.
An approach similar to the TCP freeze principle is used by a solution proposed by document [4]. In this solution, the BTS sends an explicit feedback on the bad state of wireless link to the TCP source residing in the fixed network to reset all timeout timers. For this purpose, document [4] proposes to implement an Explicit Bad State Notification (EBSN) ICMP message. However, the EBSN mechanism according to document [4] requires the implementation of a new ICMP message while the solution according to the present invention does not require the definition and implementation of new message types of any kind. Moreover, the EBSN requires extended TCP functionality, which limits the usability of the solution proposed in document [4]. The solution according to the present invention requires no additional TCP functionality; its operation is transparent to the TCP layer.
A different approach than the TCP freeze principle is used by document [5] to avoid spurious TCP timeouts during handovers in LTE networks. Instead of freezing TCP sources, the two methods proposed by the authors are based on the idea of decreasing the amount of forwarded packets.
Contrary to the solution according to the present invention, both methods require the modification of the standardized handover message sequence (TS 23.401, TS.36.413) as the path switch is done earlier than the reception of the handover confirm message in the target eNB. Additional drawback of these solutions is that carrying out the path switch before the handover is confirmed introduces the risk of inconsistent system state upon unsuccessful handover. As TCP traffic is not stopped in case of handover and limited radio conditions, the effect of packet losses not related to congestion cannot be mitigated by these solutions.